Process for detecting gamma rays



June 24, 1952 A. J. F. SIEGERT PROCESS FOR DETECTING GAMMA RAYS OriginalFiled June 22, 1945 IN V EN TOR.

7 TORNEY? Patented June 24, 1952 PROCESS FOR DETECTING GAMMA RAYS ArnoldJ. F. Siegert, Evanston, Ill., assignor to The Texas Company, New York,N. Y., a, corporation of Delaware Original application June 22, 1945,Serial No. 600,865. Divided and this application March 5, 1949, SerialNo. 79,773

Claims. (Cl. 250-83.6)

This invention relates to the measurement of penetrative radiation andmore particularly to the measurement of relatively low energy radiationsuch as scattered gamma rays. The prinmetals of lower atomic number whenmeasuring direct gamma radiation, the increase in efiiciency of thehigher atomic number metals is proportionately much greater whenmeasuringlow encipal object of the invention is the provision of s erggygamma rays such as those which have been a device for detectingradiation of comparatively scattered in an object or medium than whenlow energy such as gamma rays originating in a measuring higher energyrays such as the direct source and scattered within a medium or media orprimary rays passing directly from the source back to the detector,which detecting device has to the detector. This has been proven experiahigher efficienc than those commonly in use. mentally and tests madewith detectors having This is a division of my copending applicationbrass cathodes and detectors having lead cath- Serial No. 600,865, filedJune 22, 1945, now abanodes have shown that while in measuring direct dned, radiation there is an increase in emciency due In the U. S. LettersPatents of D. G. C. Hare, to the lead or" say 28 to 30%, the increase inef- Nos. 2.2771156, 2,348,810, 2.304,010, and in the 00- ficiency of thelead in measuring scattered pending application of Herzog et al., SerialNo. gamma is frequently from one hundred to 574,870, filed January 27,1945. now Patent No. several hundred per cent, 2,536,131, instrumentsare disclosed for making The advantages obtainable due to this increasedvarious measurements such as the thickness of a efficiency in m eCharacteristics ch plate or t Wall, t lever f a liquid i quire thedetecting of scattered radiation will be tainer and the density of afluid in a container apparent. Thus, in an instrument such as is disorpipe entirely from one side of the object or closed in theaforementioned Hare Patent No. container, In each case gamrng, raysfrom, 3, 2,277,755 1316i ill. the conending Herzog 81] a1. alpsource arecaused to. penetrate an object or me- D a i- Serial No. 574,87. greaterthicknesses dium and a portion of the rays scattered in and of an objectcan be accurately measured with returned outwar ly of the object ormedium are the same amount of radioactive source. or the measured, thisIneasurement providing an jndiga- 33.2?(18 thicknesses C953. beIZISELSUI'GCI With a smaller tion of the characteristic desired. Inorder to source. Likewise, with certain amount of radioobtain useful andaccurate measurements it is, active source a thickness measuringinstrument of course, desirable that the instrument used for in whichthe detector cathode is formed of lead, measuring t scattered rays h asvhigh an for instance, can be moved more rapidly over a emciency aspossible, In instruments of thi type surface of the well being measuredthan an in m primary i t rays 8 those itt d strument with the same sizesource but having directly from the source also strike the deteca e rCathode of copp r a s a d 00ntor thus causing false readings to beobtained. seqllelltly more measurements can be made in a It is oftendifficult to minimize the direct rays Specified Again, in Certainmetllfidsl 0f' 0gto the desired extent without providing large t theformat-5811s S undin a Well or bore masses of shielding material betweenthe source 11016, ay fr m a source in the hole are and the detector, andnaturally the larger the caused to penetrate the surrounding formationsShiem t farther t t t t be spaced wherein. of the rays are scatteredback to a f m the source It 15: therefore very desirable detector in thevicinity of the source. When usto provide a detector which will have agreater s a detector more efiicient in measuring s atpmportionalresponse to the Scattered rays which tered gamma rays than in measuringdirect it is desired to measure than to the direct rays am a r s the ntr m nt an be passed more which t is desired t minh'nizg 5 rapidlythrough the hole with an attendant sav- In accordance with the inventionit has been ing 1111119- found that metals of high. atomic number such1T1 f ewi s, several different forms of as lead, tungsten and the like,when used as the de ectors will be described, each of cathode of thegamma ray detector show a much these dctfctors being provided with acathode of higher efficiency than metals of lower atomic a metal havinga h gh atomic number, prefernumber such as brass and copper when thegamma rays to be measured. have an energy of 0.5 megavolt or less. thisbeing due to the photoeffect. Although cathc, of the higher atomicnumber metals are also more eificient than the ably between 73 and 83,Because of the comparative cheanness of lead, this metal will bereferred to gene if] but it is to understood that other metals such, forinstance, tungsten and gold, will also be very satisfactory, as willanyof the metals in the following list of elements having atomic numbersbetween 73 and 83:

Tantalum '73 Platinum s- 78 Tungsten '74 Gold 79 Rhenium 75 Thallium 81Osmium 76 Lead 82 Iridium '77 Bismuth 83 For a better understanding ofthe invention reference may be had to the accompanying drawing in which:

Figure 1 is a somewhat diagrammatic representation of a gamma raydetector or counter of a more or less conventional type;

Figure 2 is a similar representation of another embodiment of theinvention;

Figures 3 and 4 are respectively transverse and longitudinal sectionsthrough the device of Figure 2;

Figure 5 is a diagrammatic representation of another form of detector inwhich the anode is disposed at right angles to a bank of cathode plates,and

Figure 6 is a curve showing the outputs of two detectors used inmeasuring the thickness of steel plates, one of the detectors having alead cathode and the other having a brass cathode.

In Figure 1 is shown a radiation detector or counter of a conventionaltype, this device con sisting of a thin walled metal tube in! with athin wire comprising the anode l2 disposed on the longitudinal axis ofthe tube I0 which forms the cathode. These electrodes are shown asenclosed in a suitable sealed envelope or casing US which may be a glasstube or a metal container and which contains a suitable gas such as amixture of argon and petroleum ether at a fairly low pressure of, forinstance, cm. of Hg. A cen tral wire or anode I2 is preferablymaintained at a positive potential with respect to the oathode I!) and afairly high resistance R is connected in series with a source [6 of apotential generally around 900 to 1000 volts. A gamma ray striking thecathode Ii! may eject an electron therefrom which in turn may ionize thegas causing a discharge to take place with a current flow of the orderof a few microamperes. This causes a large voltage drop across theresistance R and by suitably amplifying this voltage drop by means of anamplifier IS, a mechanical recorder 20 or other device capable ofregistering the discharges of the counter may be actuated. The cathodecylinder ii is formed of a thin sheet of about .016 inch in thickness ofa metal having an atomic number between 73 and 83 since because of thehigh atomic weight and high density of these metals there is a muchgreater likelihood of electrons being ejected by gamma rays of lowenergy than in the case of metals such as brass and copper.

In Figures 2, 3 and 4, a somewhat different form of detector or counteris illustrated in which the cathode is formed of a bank of thin leadsheets or plates Illa spaced uniformly apart. Midway between eachadjacent pair of cathode plates a plurality, three in this instance, offine wires are stretched so as to be parallel to each other and to theplates. The electrodes are mounted within a suitable casing,

or envelope Ma as was described with reference to Figure 1 and theplates are connected together electrically to form the cathode of thedevice. The wires |2a may all be connected together to form a singleanode or the wires may be connected in separate groups to form a numberof electrically separated anodes. The wires [2a being disposed in thismanner provide the desired concentration of the electrical field and itis to be understood that while only three cathode plates and two sets ofanode wires are illustrated, any greater number may be used, this numberbeing limited only by the size of the surrounding casing or envelope.

In Figure 5 still another form of detector or counter is illustrated,this device comprising a plurality of thin, circular, lead sheets orplates 22, these plates being disposed in separated parallel relationand connected together electrically to form a cathode. Each plate 22 isprovided with a plurality of holes 24 shown in this instance as four innumber, and the holes in the bank of plates are arranged in alignment inseveral groups or series. Through each series of holes 24 a fine wire 25is stretched on the axis of the series of holes and the wires are shownas connected together electrically to form the anode of the device. Inthis instance and for the sake of clearness, no casing has been shown,but it is to be understood that as many of the plates 22 as desired willbe arranged in an elongated, preferably cylindrical casing of brass orother suitable material, the casing being filled with a suitable gas asdescribed with reference to Figure 1.

Several detectors or counters of the type shown in Figure 5 have beenconstructed and found very satisfactory. As a typical example, one ofthese counters contains a stack or bank of cathode sheets or plateswhich are spaced from each other a distance of of an inch. Each platecontains four holes /2 inch in diameter and through each of the holes atungsten anode wire of 3 mil diameter is disposed. Twelve lead cathodeplates, each of .016 inch thickness and 2 inches in diameter, aremounted in a cylindrical brass casing four inches in length. Counters ofthis type have been used effectively in the instruments described in theaforementioned-patents and copending patent application.

In Figure 6 is shown a curve or rather a pair of curves which were madeon two of the counters as described in the above paragraph, one of thecounters having a lead cathode and the other a brass cathode, both ofthese counters being used to measure scattered gamma rays, i. e., rayshaving an energy of 0.5 megavolts or less. These curves were obtainedunder exactly the same geometrical conditions and with the same amountof radium. It will be noted that the response of the lead cathodecounter was substantially twice as great as the response of the brasscathode counter.

It has been found advisable to clean the surfaces of the lead sheets orplates before use and the following method has proven very satisfactory.The lead plates are first dipped for several minutes in aqua regia andthey are then immersed for approximately one minute in a boilingconcentrated solution of sodium hydroxide. The plates are then rinsedwith water, alcohol and ether and dried in the open air. After theassembly of the counter, it is filled with a mixture of substantially97% argon and 3% pctroleum ether, the filling process consisting inevacuating the counter with a rotary pump for about three hours withoutheating the counter. During this process the counter is flushed severaltimes with argon. The filling pressure of the final gas is the same asfor brass cathode counters, about 20 inches absolute pressure. Tungstenis also an effective metal for these counters when measuring low energygamma rays and since tungsten is chemically inactive, it is thereforeeasy to maintain a clean metallic surface. A commercially obtainablematerial called Mallory 1000 contains over 99% pure tungsten and canreadily be formed into thin sheets of the desired shape and size.

Obviously many modifications and variations of the invention, ashereinbefore set forth, may be made without departing from the spiritand scope thereof and, therefore, only such limitations should beimposed as are indicated in the appended claims.

I claim:

1. In a process involving detection of gamma rays in a detector havingan anode and a cathode with an ionizable medium disposed between them,the improvement which comprises introducing at least some of the gammarays having an energy above a level of about 0.5 megavolts into a secondmedium in which at least some of the introduced gamma rays are partiallydeenergized below said energy level, introducing at least some of thegamma rays thus deenergized and some of the gamma rays having an energyabove the 0.5 megavolt level into the detector, and providing thecathode thereof with a metal of a high atomic number which imparts tothe detector as compared with one provided with a metal cathode of lowatomic number an increase in detection efficiency which isproportionally greater for gamma rays below about the 0.5 megavolt levelthan for gamma rays above said level, whereby the detection of the gammarays below about the 0.5 megavolt level is emphasized while thedetection of the gamma rays above the level is deemphasized.

2. In a process involving detection of gamma rays in a detector havingan anode and a cathode with an ionizable medium disposed between them,the improvement which comprises introducing a beam of gamma rays atleast some of which have an energy above the 0.5 megavolt level into asecond medium in which at least part of the gamma rays areback-scattered and are deenergized below said level, introducing thebackscattered gamma rays of the energy level below 0.5 megavoltstogether with gamma rays which have not been back-scattered and have anenergy level above 0.5 megavolts into the detector, providing thecathode thereof with a high atomic number metal which imparts to thedetector. as compared with one provided with a metal cathode of lowatomic number, an increase in detection efficiency which isproportionally great er for gamma rays below about the 0.5 megavoltlevel than for gamma rays above said level, whereby the back-scatteredgamma rays are detected with higher efficiency while discriminatingagainst the detection of the gamma rays of higher energy.

3. Process according to claim 2 in which the cathode is of tantalum.

4. Process according to claim 2 in which the cathode is of lead.

5. Process according to claim 2 in which the cathode is of bismuth.

ARNOLD J. F. SIEGERT.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Name Date Hare Mar. 19, 1946 OTHER REFERENCESNumber

1. IN A PROCESS INVOLVING DETECTION OF GAMMA RAYS IN A DETECTOR HAVINGAN ANODE AND A CATHODE WITH AN IONIZABLE MEDIUM DISPOSED BETWEEN THEM,THE IMPROVEMENT WHICH COMPRISES INTRODUCING AT LEAST ONE OF THE GAMMARAYS HAVING AN ENERGY ABOVE A LEVEL OF ABOUT 0.5 MEGAVOLTS INTO A SECONDMEDIUM IN WHICH AT LEAST SOME OF THE INTRODUCED GAMMA RAYS ARE PARTIALLYDEENERGIZED BELOW SAID ENERGY LEVEL, INTRODUCING AT LEAST SOME OF THEGAMMA RAYS THUS DEENERGIZED AND SOME OF THE GAMMA RAYS HAVING AN ENERGYABOVE THE 0.5 MEGAVOLT LEVEL INTO THE DETECTOR, AND PROVIDING THECATHODE THEREOF WITH A METAL OF A HIGH ATOMIC NUMBER WHICH IMPARTS TOTHE DETECTOR AS COMPARED WITH ONE PROVIDED WITH A METAL CATHODE OF LOWATOMIC NUMBER AN INCREASE IN DETECTION EFFICIENCY WHICH ISPROPORTIONALLY GREATER FOR GAMMA RAYS BELOW ABOUT THE 0.5 MEGAVOLT LEVELTHAN FOR GAMMA RAYS ABOVE SAID LEVEL, WHEREBY THE DETECTION OF THE GAMMARAYS BELOW ABOUT THE 0.5 MEGAVOLT LEVEL IS EMPHASIZED WHILE THEDETECTION OF THE GAMMA RAYS ABOVE THE LEVEL IS DEEMPHASIZED.